feat(tax): universal tax fallback — implement as 'Apply to all countries' country dropdown option

[superdav42]

Background

PR #277 (feat(tax): add universal tax fallback feature) was closed without merge. The maintainer (@superdav42) requested a different implementation approach.

Requested Implementation

Instead of a separate settings toggle + rate field, implement universal/fallback tax as an 'Apply to all countries' option in the existing country dropdown on the Tax Rates configuration page.

When a user selects 'Apply to all countries', the rate applies as a fallback when no country-specific rate matches.

Original PR scope (for reference)

• PR feat(tax): add universal tax fallback feature #277 added enable_universal_tax toggle + universal_tax_rate field (0-100%)
• Maintainer feedback: use the existing country dropdown UX instead
• Branch was deleted; code needs to be reimplemented per the new approach

Acceptance Criteria

• 'Apply to all countries' option added to country dropdown on Tax Rates page
• Rate configured via this option applies as fallback when no country-specific rate matches
• Translations updated for new strings
• Existing country-specific rates take precedence over the universal fallback

[superdav42]

Completed — already implemented in main

This feature was implemented across several PRs that have all been merged:

What was done:

• PR fix(tax): redesign universal tax as 'Apply to all countries' dropdown option #427 — Initial implementation: added <option value="*">Apply to all countries</option> as the first option in the country dropdown on the Tax Rates configuration page (views/taxes/list.php)
• PR feat(tax): implement universal tax as 'Apply to all countries' option in Tax Rates page #456 — Added unit tests covering all wildcard fallback scenarios (5 test cases in Tax_Functions_Test.php)
• PR t452: feat(tax): implement universal tax fallback as 'Apply to all countries' dropdown option #506 — Formal closure of the feature with improved docblock documentation on wu_get_applicable_tax_rates()
• PR fix(tax): skip wu_get_country_states() for wildcard '*' country rates #512 — Follow-up fix: skip wu_get_country_states() for wildcard * country rates to prevent invalid lookups

How it works:

• A rate with country='*' (selected via "Apply to all countries" in the dropdown) acts as a universal fallback
• Country-specific rates always take precedence (priority scoring: country match = +10 points; wildcard fallback only applied when $results is empty after the main loop)
• No new settings fields required — the existing Tax Rates page UI handles everything

Files changed:

• views/taxes/list.php — <option value="*">Apply to all countries</option> added as first option in country <select>
• inc/functions/tax.php — Fallback logic in wu_get_applicable_tax_rates() (lines 210–221): when no country-specific rates match, rates with country='*' are returned as fallback
• lang/ultimate-multisite.pot — "Apply to all countries" translation string present
• inc/tax/class-tax.php — Guard added to skip wu_get_country_states() for wildcard rates

Acceptance criteria verified:

• 'Apply to all countries' option in country dropdown on Tax Rates page
• Rate applies as fallback when no country-specific rate matches
• Country-specific rates take precedence
• Translations updated

This supersedes the rejected PR #277 (separate settings toggle approach). The maintainer's requested approach (dropdown option on the Tax Rates page) is what was implemented.